The present invention relates to an improvement of a drum brake type electric parking brake apparatus which expands a distance between one pair of brake shoes with an electric motor to produce a braking force. In particular, the present invention is to realize a simple structure which improves a smoothness of a braking when the braking is started or cancelled by smoothing an operation of a part which transmits an output of an electric actuator unit to an expanding mechanism for expanding the distance between one pair of brake shoes.
The drum brake type electric parking brake apparatus of the present invention includes a drum-in disc brake for which a service brake is carried out by a disc brake, and only a parking brake is carried out by a drum brake, as well as the structure in which drum brakes are used both in the service brake for decelerating or even stopping a vehicle from a running state and in the parking brake for keeping a stopped state.
A parking brake which maintains a vehicle at a stopped state while a brake pedal is not pressed is typically incorporated in a drum brake. Further, it is known from those described, for example, in a patent document 1 that in some vehicles the parking brake is performed mechanically (not hydraulically) through a boosting mechanism such as link mechanisms like a toggle mechanism while an electric motor is a driving source. In this patent document 1, a structure is described in which a hydraulic disc brake is used as the service brake, and a mechanical drum brake, in which an electric motor is a driving source, is used as the parking brake.
The drum brake type electric parking brake apparatus described in the patent document 1 is so configured that the distance between a pair of brake shoes, which are supported at an outer side surface of a backing plate to be displaceable in a radial direction of the backing plate, is expanded with an electric actuator unit. This electric actuator unit is made by combining the electric motor, a gear type speed reducer and a feeding screw type rotation linear movement converting device, and when the electric motor is electrified, a rod of the rotation linear movement converting device is moved by a large force in a linear direction, and a cable is pulled by the rod. The distance between one ends of the two brake shoes in the circumferential direction is expanded by this cable and an expanding mechanism such as the link mechanism.
FIG. 8 shows an electric actuator unit 1 which is considered by this inventor earlier and is different from the structure described in the patent document 1. In this electric actuator unit 1, the rotation of an output shaft 4 of an electric motor 3 installed in an actuator case 2 is transmitted to a screw pipe 6 through a gear type speed reducer 5. The screw pipe 6 is supported in the actuator case 2 to be only rotatable, and a female screw is formed on the inner circumferential surface of the screw pipe 6. A male screw formed on the outer circumferential surface of a rod 7 is threadedly engaged with the female screw of the screw pipe 6. The rod 7 is provided in the actuator case 2 to be only movable in the axial direction of the rod 7. The base end of the cable 8 is attached and fixed to the distal end of the rod 7 which projects from an opening at the distal end of the screw pipe 6. Furthermore, a bifurcated connector 10 is attached to a part projecting from a dustproof boot 9 at the distal end of the cable 8. When the electric actuator unit 1 is incorporated in a parking brake, the actuator case is supported by and fixed to a fixed part such as a backing plate or a knuckle, and the connector 10 is pivotally supported by a part of the component members of the expanding mechanism.
At the time of braking, the electric motor 3 is energized, and because of the threaded engagement of the female screw on the inner circumferential surface of the screw pipe 6 and the male screw on the outer circumferential surface of the rod 7, the rod 7 is moved linearly in a direction away from the expanding mechanism. The cable 8 is pulled by the rod 7 and the expanding mechanism is extended to expand the distance between the pair of brake shoes so that the linings of these two brake shoes are pressed against the inner circumferential surface of a drum and a braking force is produced.
In the case of the above described electric actuator unit 1, the full length of the cable 8 between the distal end of the rod 7 and the connector 10 cannot be provided enough. On the other hand, a part which is connected to the connector 10, as a part of the component members of the expanding mechanism, moves arcuately with the expansion and contraction of the expanding mechanism. Therefore, to make this arcuate movement smooth, it is desirable to lengthen the full length of the cable 8 as much as possible to ensure the flexibility of the cable 8 (easiness to flex). From this aspect, there is a room for the improvement of the structure shown in FIG. 8.
In the patent document 2, a structure is described in which a plurality of link arms are combined to absorb a difference between the linear movement of one end of a rod that constructs an electric actuator unit and the arcuate movement of a part of the component members of the expanding mechanism. According to such a structure, the difference can be absorbed sufficiently, but it cannot be avoided that the structure is complicated and the cost increases.
[Patent document 1] JP 2001-504417 A
[Patent document 2] U.S. Pat. No. 8,011,482 B